High intensity metal halide arc discharge lamps include an arc tube sealed within a light-transmissive lamp envelope. Electrical energy is coupled through a lamp stem to the arc tube. Metal halide arc discharge lamps frequently include a shroud which provides performance and safety improvements. The shroud comprises a cylindrical, light-transmissive member, such as quartz, that is able to withstand the high operating temperatures of the lamp. The arc tube and the shroud are coaxially mounted within the lamp envelope, with the arc tube positioned within the shroud.
A shroud open at one end and having a domed configuration at the other end for use in a low wattage metal halide lamp is disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et.al. and U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et.al. The shroud is suggested as being useful in reducing heat loss from the arc tube by convection and thereby raising the temperature of the arc tube and increasing the vapor pressure of the volatile metal halide additives in the arc discharge. Sodium loss is stated to be reduced when the shroud is used in a gas-filled lamp envelope.
Sodium is an important constituent in most high intensity metal halide arc discharge lamps, usually in the form of sodium iodide or sodium bromide. Sodium is used to improve the efficacy and color rendering properties of these lamps. It has long been recognized that arc tubes containing sodium lose sodium during discharge lamp operation. Sodium is lost by the movement, or migration, of sodium ions through the arc tube wall. The iodide originally present in a metal halide lamp as sodium iodide is freed by sodium loss, and the iodide combines with mercury in the arc tube to form mercury iodide. Mercury iodide leads to increased reignition voltages, thereby causing starting and lamp maintenance problems.
In U.S. Pat. No. 4,281,274 issued July 28, 1981 to Bechard et.al., a miniature arc tube containing sodium iodide is located within a gas-filled outer envelope. The arc tube is mounted within a shroud that is open at both ends. The shroud is electrically biased with a DC voltage in order to repel positive sodium ions which have migrated through the wall of the arc tube and to attract electrons produced in the lamp envelope by the photoelectric effect. This technique is not suitable for AC operation of an arc tube, since the positive bias is provided on the shroud only during one-half of the AC voltage cycle.
A prior attempt to reduce sodium loss from AC metal halide lamps was the use of a so-called "frameless construction" described in U.S. Pat. No. 3,424,935 issued Jan. 28, 1969 to Gungle et.al. In the frameless construction, there are no frame members close to the arc tube. The electrical connection to the upper electrode is a fine tungsten wire spaced as far away from the arc tube as possible. Although this configuration reduces sodium loss, sodium loss is still evident near the end of the life of such lamps.
Another technique for reducing sodium loss is disclosed in U.S. Pat. Nos. 4,620,125 issued Oct. 28, 1986 to Keeffe et.al. and U.S. Pat. No. 4,625,141 issued Nov. 25, 1986 to Keeffe et.al. A low wattage metal halide discharge lamp includes an evacuated envelope containing a heat reducing member and an arc tube within the heat reducing member. The heat reducing member and the arc tube have a metal band and an outer strap adjacent to one another and adjacent to one electrode. The metal band, outer strap and electrode are all electrically connected to an electrical lead of one polarity, whereby sodium loss from the arc tube is reduced.
Other techniques for reducing sodium loss from arc discharge lamps are disclosed by Keeffe et.al. in Journal of Illumination Engineering Society,Summer 1988, pages 39-43; U.S. Pat. No. 4,963,790 issued Oct. 16, 1990 to White et.al.; Japanese Patent No. 60-40138 published July 30, 1976 and U.S. Pat. No. 4,843,266 issued June 27, 1989 to Santo et.al.
In the aforementioned U.S. Pat. Nos. 4,499,396 and 4,580,989, two techniques are disclosed for mounting the shroud in the lamp. In a first technique, the shroud is held in place by two shroud straps which are welded to a supporting frame. Straps positioned around each end of the arc tube are also welded to the frame and thereby support the arc tube. In a second technique, slots are cut in the shroud, and the shroud is held in place by the straps which support the arc tube. Although these lamps perform generally satisfactorily, the shroud straps permit excessive axial movement of the shroud during shipping and handling, and the slotted shroud tends to crack during manufacturing and operation.
These issues are addressed in pending application Serial No. 07/539,752 filed June 18, 1990. The disclosed mounting arrangement includes a frame comprising one or two support rods, and upper and lower clips for retaining the shroud and the lamp capsule. The clips, which are welded to the support rods, prevent both axial and lateral movement of the shroud. The frame is attached to the base end of the lamp envelope by a strap which encircles the lamp stem.
Although the lamps disclosed in application Serial No. 07/539,752 are mechanically strong and relatively simple to construct and are able to survive shipping and handling without significant breakage, these lamps have been found to have a shorter operating life than is known to be achievable. The shorter operating life is due primarily to an excessive rate of voltage rise and changes in the color temperature of the lamp during operation, which are indicative of sodium loss. One technique that has been used to increase the life of these lamps is to place a strip of insulating material under the strap which secures the frame to the lamp stem. The purpose of the insulating strip is to reduce leakage currents between the frame and the inleads to the arc tube. Although the insulating strip has been found to increase the life of the lamp, this modification is expensive to implement, increases the number of rejected lamps during the manufacturing process and does not increase the operating life to the extent desired.
Resilient bumpers, or bulb spacers, are used to stabilize an arc tube structure within a lamp envelope in the aforementioned U.S. Pat. No. 3,424,935. A spring extending between the dome end of a lamp envelope and the dome of a shroud is disclosed in the aforementioned U.S. Pat. No. 4,499,396. However, in all prior art known to applicant, the arc tube is generally rigidly mounted within the lamp envelope, and springs or other resilient members are used to supplement the rigid support and to stabilize the arc tube in a desired position.
It is a general object of the present invention to provide improved arc discharge lamps.
It is another object of the present invention to provide arc discharge lamps wherein voltage rise and changes in color temperature during the operating life of the lamp are limited.
It is a further object of the present invention to provide arc discharge lamps wherein sodium migration from the arc tube is suppressed.
It is yet another object of the present invention to provide arc discharge lamps which are capable of withstanding mechanical shock and vibration.
It is still another object of the present invention to provide arc discharge lamps which have long operating lives.
It is a further object of the present invention to provide arc discharge lamps which are simple in construction and low in cost.